(1) Field of the Invention
The invention relates to a method of metallization in the fabrication of integrated circuits, and more particularly, to a method of polishing a metal layer to form a metal plug without dishing in the manufacture of integrated circuits.
(2) Description of the Prior Art
Tungsten-plug metallization is well-known in the art. A conventional tungsten plug process of the prior art is illustrated in FIG. 1. Referring now to FIG. 1, there is shown semiconductor substrate 10. A contact, via, or trench is patterned into an oxide layer 20 to an underlying region 11. A layer of tungsten 26 is deposited which will form a plug within the opening. Typically, barrier metal layer 24 is deposited underlying the tungsten layer 26. The tungsten and barrier metal layers are then polished using chemical mechanical polishing (CMP) to remove the excess metal material, stopping on the oxide layer, as shown in FIG. 2. Because of the delayed response time of the oxide endpoint detector, some oxide overpolishing occurs. Since the oxide layer has a lower CMP removal rate than tungsten, the oxide overpolishing will result in dishing 27 of the tungsten plug. In some dense areas of some dies, some of the tungsten and barrier metal is not completely polished away when oxide overpolishing occurs in some other areas. Dishing and tungsten/barrier metal residues remaining after CMP can adversely affect the electrical performance of the integrated circuit device. It is desired to prevent dishing after tungsten CMP and to improve contact resistance between the tungsten plug and the metal stack interconnect.
Co-pending U.S. patent application Ser. No. 09/110,419 to Sudipto R. Roy filed on Jul. 6, 1998 discloses a sacrificial or semi-sacrificial titanium nitride layer deposited over the oxide to protect the oxide and to act as an endpoint detector. Polishing rates of the tungsten and the titanium nitride are comparable, resulting in dishing. U.S. Pat. No. 5,578,523 to Fiordalice et al teaches the use of a polish assisting layer over a dielectric layer and under a metal layer deposited within a trench. The polish assisting layer and the metal layer are polished at close to the same rate in the final stages of polishing, thus preventing dishing. U.S. Pat. No. 5,798,302 to Hudson et al teaches a low friction layer under a metal layer wherein the polishing rate of the low friction layer is much lower than that of the metal layer causing the CMP process to stop at the top surface of the low friction layer. U.S. Pat. No. 5,886,410 to Chiang et al discloses a hard mask over a polymer through which a trench is etched and filled with tungsten. The tungsten is polished with a higher selectivity to tungsten than to the underlying hard mask. U.S. Pat. No. 5,854,140 to Jaso et al teaches a metal stop layer under an aluminum trench filling layer. The aluminum is polished to the stop layer, resulting in dishing. Then the stop layer is removed with a very high selectivity to the stop layer over the aluminum layer so that the resulting aluminum is substantially planar. U.S. Pat. No. 5,776,833 to Chen et al teaches a titanium nitride layer under a tungsten plug layer. CMP stops at the titanium nitride layer. The titanium nitride layer is then removes by etching rather than by polishing resulting in a protruding tungsten plug. U.S. Pat. No. 5,356,513 to Burke et al discloses alternating layers of soft polishing material and hard polish stops to provide tungsten plugs having a substantially planar surface.